This invention relates generally to infrared spectrometer detectors in general and is particularly directed to a liquid sample flow cell for directly interfacing with a microbore high performance liquid chromatography/Fourier transform infrared spectrometer.
The use of a Fourier transform infrared (FTIR) spectrometer as a detector for high performance liquid chromatography (HPLC) is well known in the art. Recently, microbore HPLC columns have been used in combination with FTIR spectrometers. Microbore HPLC spectrometers offer three primary advantages over conventional FTIR detectors. First, the substantial decrease in required sample, e.g., approximately 20-fold, less in microbore columns below analytical scale columns substantially enhances the detectability of species. This is particularly important when working with limited sample sizes. In addition, the low solvent consumption of microbore columns allows for the economical testing of various potential IR solvents which would otherwise be cost prohibitive. Finally, the microbore HPLC approach offers high detection limits and the possibility for individual compound separation and identification by FTIR of very complex mixtures without the previous associated large solvent consumption.
Referring to FIG. 1, there is shown an exploded view of a prior art sealed, parallel plate interface flow cell 10 for use in microbore HPLC/FTIR systems. The parallel plate interface flow cell 10 is comprised of a first window 12, a second window 22 and a metal spacer 14 interspaced therebetween. The first and second windows 12, 22 are typically comprised of potassium bromide (KBr) and the thickness of the metal spacer 14 establishes the IR pathlength through the interface flow cell. In this arrangement, the sample as well as the effluent are introduced into a first aperture 24 in the second window 22 and flow through the "keyhole" arrangement of a first flow channel 18, a center aperture 16 and a second flow channel 20 within the metal spacer 14 and exit through a second aperture 26 within the second window 22. An IR beam is directed through the first and second windows 12, 22 and through the center aperture 16 of the metal spacer 14 for providing a spectral analysis of the sample flowing therethrough. The primary disadvantage of this interface flow cell arrangement is the circuitous sample/effluent flow path of the sealed cell and its complexity in terms of the various gaskets and tubing necessary for connection. The unswept or mixing volume is exhibited in chromatographic measurements as a chromatographic peak characterized by a tailing response. In addition, the off-axis location of the first and second apertures 24, 26 within the second window 22 relative to the first and second flow channels 18, 20 in the metal spacer 14 produces a region of dead volume in which eddy currents are generated resulting in the aforementioned chromatographic peak exhibiting a tailing response.
In spite of these limitations, the prior art seal interface flow cell 10 shown in FIG. 1 was believed to be the optimum design for interfacing with a liquid chromatography/Fourier transform IR spectrometer because it provided a single path length for the IR beam and thus obeyed Beer's law with respect to the absorption of light traversing a given cell length. It was thought that a non-planar interface cell configuration would not obey Beer's law and thus would not afford consistent and predictable IR beam absorption characteristics.
The present invention is intended to avoid the aforementioned limitations of the prior art by providing a liquid chromatography/Fourier transform IR spectrometry interface flow cell which essentially eliminates stagnate or eddy currents in the sample/effluent flow path for more accurate measurements. In addition, the circular cross-section of the liquid sample serves as a focusing lens due to the refractive index differences between the solvent and the interface flow cell crystal structure in minimizing interference patterns and vignetting observed in conventional parallel-plate infrared cells.